Machinery often comprises parts, which, during normal operation, would be hazardous to an operator should the operator come into contact with those parts when they are moving.
Such machinery is often driven by an electric motor. For safety reasons, it is often a requirement that a control system be employed for allowing and preventing operation of the electric motor (and hence machine operation) with a high level of integrity. For example, when a safety guard or gate is opened to allow access to a part of a machine that would be hazardous when moving, the motor must be prevented from driving the machine. A typical level of integrity for such a function would be a probability of dangerous failure on demand of the order of 10−8 per hour. To achieve this, circuit design is employed that ensures that most component failures and combinations of failures result in the motor being prevented from driving the machine and, in turn, the machine not operating.
Traditionally, the ability to enable or disable the operation of the electric motor is achieved with electromechanical contactors, at least two of which would be arranged in series with the motor. The contactors are typically provided with auxiliary monitoring contacts so that an incorrect position of the main contacts of one contactor could be detected, and completion of the circuit prevented by disconnecting both coils of the electromagnets of the contactors.
Recently, solid-state controllers that drive an inverter to convert the d.c. supply into a phased set of a.c. supplies to produce a rotating magnetic field in the motor have been equipped with safety-related inputs. The inputs allow the operation of the motor to be prevented by electronic means.
In order to maintain torque in the motor, continual active and co-ordinated switching in the required sequence of the corresponding power semiconductors is needed. Should erroneous conduction of one or more of the power semiconductor devices of the inverter occur, this does not result in sustained torque in the motor. For a motor with a smooth (non salient) rotor, no torque is produced by any failure of a power semiconductor device of the inverter. For a motor with permanent magnets and/or saliency, a pair of short circuit power semiconductor devices in the inverter could cause a brief alignment torque whereby the motor partially rotates, however, the current would increase rapidly until interrupted by a protection device (for example a fuse) or destructive failure of at least one of the power semiconductor devices.
As a further example, in power grid-connected power generating inverter applications, the same principles apply when the inverter drives a transformer rather than a motor. Erroneous conduction of power semiconductor devices of the inverter cannot produce an alternating flux in the transformer, and therefore cannot produce a sustained output from the transformer secondary coil. In other words, a fault in the inverter power device results in direct current, which cannot be transferred through the transformer because the transformer relies upon alternating current for its operation.
In order for safe and reliable control of such an inverter, an interface is required between the inverter control input terminals which typically use logic signals such as 24V d.c. and the power semiconductors of the inverter that maintains the required low probability of dangerous failure of the inverter.
Electromechanical relays have been used to provide the necessary electrical isolation and electrical level conversion for such an interface. However, relays possess relatively high probabilities of failure in the dangerous direction and have a relatively short time before mechanical wearout. This results in pairs of relays being used accompanied by monitoring to detect fault conditions.
Recently, generation of the power semiconductor control signals for operating the inverter is typically carried out by complex digital electronic circuits and programmable digital processors. Such an arrangement does not provide the required low probability of dangerous failure as most digital circuits can fail with equal probability into either of the available logic states. Further, the complexity of the digital circuits and functions is such that it is difficult to reliably and confidently demonstrate a sufficiently low probability of dangerous failure under all combinations of conditions and sequences of conditions that the circuit may be subjected to during operation. For example, it may be difficult to predict how the circuit reacts under changeable temperature conditions together with each and every possible sequence of combinations of logic levels on each and every pin of the various devices of the circuit.
If complex digital electronic circuits and programmable circuits are to be employed in safety critical functions, typically, at least two independent channels together with diagnostic and cross-checking functions to detect faults or errors are used. These systems allow the disabling of an inverter by way of a channel that is not affected by a particular fault that has been detected. As can be seen, even in such systems, means for disabling the inverter which do not rely on the complex circuits needs to be provided in order to achieve the required low probability of dangerous failure.
It is therefore desirable to have a fail-safe interface, in particular, to an inverter, which employs simple electronic components with well-defined failure modes. In such an interface, it is desired that a very high fraction of component faults, and combinations of component faults, result in a safe failure. In other words, a failure where the inverter is not provided with the required waveform, and hence a motor connected to the inverter is not driven.
The same approach applies to power generators using inverters, in cases where under certain conditions, it is necessary to prevent the operation of the inverter with a high level of integrity. This could be, for example, when the part of a public power distribution network fed by an inverter has become separated from the main body of the power network and must be disabled.